1. Field of the Invention
The present invention relates to a sensing system for coordinate measuring equipment, and more particularly, to a sensing system with a contact body for contacting the surface of a workpiece to be measured.
Sensing systems for coordinate measuring equipment for tactile measurement of workpieces may be divided into two classes: sensing heads of the so-called measuring type and those of the switching type. While sensing heads of the measuring type constantly provide a signal which is proportional to the deflection of the resiliently mounted tracer pin, and are thus also suitable for the continuous tracing of workpiece surfaces in so-called scanning operation, sensing heads of the switching type produce a pulse signal on contact with the workpiece. In this latter case, the workpiece surface must be newly contacted at each point at which its position or shape is to be determined.
Switching sensing systems usually have an electromechanical switch contact in the mounting for a resiliently received tracer pin. This switch contact is adjusted very sensitively and opens as soon as the tracer pin, on contact with the workpiece, moves relative to the portion of the sensing head which is fixed to the machine.
To increase the sensitivity of such sensing heads, it has also already been proposed to build highly sensitive, e.g., piezoelectric, sensors into the tracer pin carrier or into the tracer pin itself, in order to be able to detect the contact with the workpiece as soon as there is the smallest reaction force between the workpiece and the contacting shaped body, that is, the sensing ball. Such a sensing head is described, for example, in German Patent DE-PS 27 12 181 (U.S. Pat. No. 4,177,568) and German Laid-Open Patent Applications DE-OS 28 20 813, DE-OS 33 09 122. Nevertheless, even with these sensing heads, the instant of contact cannot be reliably and precisely determined. There are several reasons for this. Firstly, a delay time occurs, particularly with long tracer pins, between the first contact with the workpiece and the response of the sensor, due to the travel time of the pressure wave or sound wave from the contacting shaped body to the sensor, dependent on the materials used for the tracer pin or its extension, etc. Furthermore, the sound wave produced on contact can be attenuated by destructive interference on its path to the sensor. This becomes in particular noticeably disadvantageous, for example, in sensing weak materials, when only a very small amount of mechanical impact energy is transmitted. Instead of perceiving the sound wave, the sensor first perceives the static pressure which builds up very much later, and which arises when the sensing head travels further on the workpiece surface.
It has also already been proposed to arrange the sensor in the neighborhood of the contacting shaped body (FIG. 8 in the first-mentioned DE-PS 27 12 181). However, such tracer pins have not yet been reduced to practice. This is because, on the one hand, the wiring and reliable contacting arrangements for tracer pins, which as a rule are interchangeable, gives rise to noticeable signal conduction problems; and on the other hand, several tracer pins are often built into complex sensor configurations on the sensing head and cannot be immediately connected in parallel with the signal leads of the sensors. The sensor is therefore usually arranged on the sensing head side of the place for change-out of tracer pins, and solutions for the problems relating to the propagation delays of sound and to signal intensity are found in other ways.
Furthermore, it has been proposed to determine the instant of contact precisely with interferometric sensors which measure a mirror surface on the contacting shaped body through the hollow tracer pin, as is described, for example, in German Patent DE-PS 39 20 716 and German Laid-Open Patent Application DE-OS 42 44 240. However, this solution is obviously not particularly well suitable for either interchangeable tracer pins or for the simultaneous attachment of several tracer heads to the sensing head of a coordinate measuring device.
Finally it is also known to measure workpieces directly on the processing machine, in that a sensing head is inserted, instead of the processing tool, in the Morse cone of the tool holding fixture. As a rule, battery-powered, self-contained sensing heads are used for this purpose, since no electrical leads for current supply and signal transmission are provided in this location. With such sensing heads, the sensing signal is then transmitted from the sensing head to the machine tool by radio or infrared radiation or even inductively. Such connecting sensing heads for machine tools are described, for example, in German Laid-Open Patent Application DE-OS 34 22 102 and U.S. Pat. Nos. 4,608,763; 4,509,266; 4,578,874; and 4,670,989. However, all of the sensing heads described therein use a switching contact located in the mounting of the moving tracer pin to generate the contact pulse, which is then transmitted further by radio means. The problems mentioned hereinabove relating to the distance between the sensor and contacting shaped body are neither solved nor addressed, since the requirements for sensors used on machine tools are basically set lower as regards contact accuracy than for sensing heads which are used on coordinate measuring devices.